My boat originally came with an 8HP 2 stroke motor. That was later replaced with a 4 stroke 8HP. The 4 stroke is much heavier than its 2 stroke predecessor. The motor sits in a well and so can't be tilted up to get it out of the water. At nearly 100 pounds, taking it out after each use is a veritable impossibility. At 60 lbs, a 6HP is much more manageable.
But, will it push the boat against a headwind?
A lot depends on your conditions. I had a Coronado 25, which also had a well, at about 4,500 lbs loaded with a 6HP 2 stroke. It was OK in calm conditions but in my opinion somewhat marginal for San Francisco Bay and outside ocean conditions. Changed to a 15HP 2 stroke, which was the same size and weight as a 9.9HP, and had all the power I could use. A 9.9 HP would have probably been enough? If you re-power suggest you get the longest shaft that will fit and a "high thrust" prop.